Mammograms lead to overdiagnosis, have little impact on deaths: study

Mammograms are having only a small impact on breast cancer death rates and have led to more than one million women being “overdiagnosed” with the disease over the past 30 years, according to a sweeping new U.S. study.

The research, which is bound to be controversial among both patients and physicians who view mammography as a major form of defence against breast cancer, found that the disease was over-diagnosed in 1.3 million U.S. women between 1976 and 2008. This means the mammograms detected tumours that would never have developed into full-blown disease.

The study also concluded that mammography had little impact on reducing the number of cases of late-stage breast cancer, and is having “at best, only a small effect on the rate of death from breast cancer.”

The researchers, from the Dartmouth Institute for Health Research in New Hampshire, note mammography has led to a large increase in detection of the number of cases of early-stage cancer, from 112 to 234 cases per 100,000 women.

But study author Dr. Gilbert Welch said the findings show that mammography fails at one of the key prerequisites of an effective screening program in that it does not decrease the number of late-stage breast cancer cases.

“It is showing screening is having no effect on metastatic cancer,” Welch told CTV News. “That’s the stage we’d most like to prevent, because they are most likely to die of the disease.”

The findings are published in the New England Journal of Medicine.

According to Welch, recent studies have shown that death rates from breast cancer are falling. However, they conclude that that is likely due to better treatments for the disease, not screening.

Mammography’s biggest success is “is detecting a whole new fraction of patients who would otherwise never be sick from their disease,” Welch said, and that overdiagnosis has its own negative side effect: unnecessary treatment.

Canadian researchers first voiced concerns about the efficacy of mammography in 1992, but were heavily criticized and largely ignored.

“This is no doubt one of the most contentious issues in medicine, and it seems that mammography is over-promoted,” Welch said. “There is no other test in medicine that has been as aggressively sold as mammography, and we communicate an unbalanced view. We over-state the benefits and we understate or ignore its harms.”

The concerns about mammography echo the controversy that has erupted over the PSA test for prostate cancer, which some researchers expressing concerns about over-diagnosis and unnecessary treatment.

Dr. Derek Muradali, chief radiologist of the Ontario Breast Screening Program at Cancer Care Ontario, told CTV News that while he finds the study “interesting,” he has “some concerns with some of their assumptions.”

Muradali points out that overdiagnosis occurs throughout medicine, and says he disagrees with the prevalence of overdiagnosis in breast cancer that the study proposes.

Muradali also says that doctors cannot be certain which cancers will become harmful, “so all cancers should be treated.”

“That’s something your viewers should understand,” Muradali told CTV. “If they have a breast cancer diagnosis picked up on screening, they need to be treated.”

Study author Dr. Archie Bleyer said there is “no question” that the research will upset many in the cancer care community.

He says that the findings point out that patients must be better informed about the benefits and risks of mammography because “every mammogram you do runs the risks of false diagnosis or over diagnosis.”

But the findings also mean researchers must continue to look for better screening methods for breast cancer.

“There is a genuine movement to reduce the amount of treatment that comes with a positive biopsy,” Bleyer told CTV. “We have to find a better way of being more accurate.”

With a report from CTV’s medical specialist Avis Favaro and producer Elizabeth St. Philip